The brain maintains caloric balance by regulating food intake and energy expenditure. To understand how this occurs, it is imperative not only to identify the body- weight regulating neural circuits, but how these neurons are connected, both anatomically and chemically. A subpopulation of neurons in the arcuate nucleus of the hypothalamus coexpress NPY and AgRP, orexigenic peptides that stimulate robust feeding when injected into the brain. Furthermore, NPY and AgRP mRNA levels increase under starvation conditions, as well as in obese animals deficient in leptin signaling, strongly implicating these neurons in homeostatic control. Despite these initial observations, genetic deletion of these genes has minimal effects on body weight regulation. Recent evidence suggests an important role for GABA, which is co-released from these neurons. However, various phenotypes have been assigned to these neurons depending on the strategy used to manipulate them, leading to a crossroads in the field. Until now, there has been no direct way to acutely control neuronal function and measure the behavioral responses in live, free-moving animals. The DREADD technology proposed here will be used to spatially and temporally stimulate/inhibit the AgRP neurons, determining their behavioral role and functional relevance in maintaining body weight homeostasis. PUBLIC HEALTH RELEVANCE: With the bleak outlook that 75% of Americans will be overweight or obese in 10 years, it is essential to find efficient means to fight this unabated epidemic. Knowledge of the powerful and complex neural systems conferring the basic drive to eat and expend energy is a prerequisite for designing effective therapies. This proposal aims to forge a more complete understanding of one of these key hypothalamic circuits regulating body weight homeostasis, the AgRP neurons of the arcuate nucleus.